The present invention is related to the field of ion implanters for use in semiconductor manufacturing.
Ion implanters used in semiconductor manufacturing include a source arc chamber in which an electrical discharge interacts with a gas to create a plasma containing a variety of ion species, including a desired species to be implanted in the surface of a semiconductor wafer. The positive ions are extracted from the source arc chamber in a known manner, and apparatus within the implanter separates the desired species from the undesired species and directs the desired species to the surface of the wafer at a desired energy level.
In one common configuration, the source arc chamber includes an emitter electrode at one end and a repeller electrode at the other end. The emitter electrode may be a cathode heated by a filament, or simply a bare filament, and its purpose is to emit electrons by thermionic emission during operation. The electrons are accelerated into the arc chamber by a relatively positive arc voltage on the arc chamber walls, and an externally generated magnetic field causes the electrons to travel a spiral path into the arc chamber. The emitter and repeller electrodes are typically biased negatively with respect to the walls of the arc chamber. The combined effect of the emitter and repeller electrodes is to concentrate electrons toward the center of the arc chamber to maximize interaction with the gas and thereby attain a desired operational efficiency.
In one known configuration, the repeller has a broad portion that faces the center of the arc chamber, and a narrower shaft that extends outside the arc chamber through an opening in the arc chamber end wall. A ceramic insulator is disposed in the arc chamber between the end wall and the repeller to maintain the required electrical isolation.
During operation, the source arc chamber contains a host of molecular species at very high temperatures. Components in this harsh environment are subjected to conditions that may unduly limit their lifetime or their effectiveness, thus limiting the effectiveness and/or increasing the operating costs of the implanter. For example, there is a tendency for films of conductive material to be deposited on the interior arc chamber surfaces, including for example the surface of the ceramic insulator on which the repeller is mounted. This coating leads to electrical breakdown, which in turn leads to burn marks referred to as “track marks” or “tracking” on the ceramic and coating. Excessive electrical breakdown can interfere with normal operation of the implanter. There are other failure modes involving deposited material and the repeller insulator as well.
It is known to remove the repeller insulator outside of the arc chamber so as to reduce the formation of a conductive film and increase the lifetime of the source. In one such configuration, the outer end of the repeller is held in place by a cantilevered arm that is secured to other structure of the implanter by an insulator component. The repeller is held in a position in which its shaft passes through the end wall opening without touching it. In this configuration, the insulator is essentially shielded from any buildup of a conductive film by the arc chamber walls themselves. An ion implanter employing such a configuration is described in U.S. Pat. No. 5,517,077 of Bright et al.